Date: Mon, 19 Oct 2015 11:18:40 -0400 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Subject: Re: musl mips n64 dynamic build On Mon, Oct 19, 2015 at 02:51:12PM +0530, Mahesh Bodapati wrote: > Hi Rich, > > I have ported musl libraries for MIPS n64 ABI architecture and I have setup > $gp as > > Crt1.s: crt1.s is obsolete and should not be used for new archs. The benefit to you is that crt_arch.h (the new way of doing it) is a lot easier to write. All you need to do is jump to the C entry point _start_c with the right calling convention for a C function and the first 2 argument slots containing the original stack pointer and a pointer to _DYNAMIC, respectively. _start_c can be assumed to be defined in the same source file. > *lui $3,%hi(%neg(%gp_rel(_start)))* > > * daddu > $3,$3,$25 /* same in dlsym.s > and pipe.s */* > > * daddiu $gp,$3,%lo(%neg(%gp_rel(_start)))* > > ld $4, %got_disp(main)($gp) # Get main() .. Jumping to main from crt1 is not valid. You need to call __libc_start_main. But if you use crt_arch.h this part happens automatically for you. > I didn’t setup $gp as in MIPS o32 ABI as we shouldn’t use _*gp*_disp for > setting up $gp > > “The special symbol name _gp_disp, used for relocating the calculation of gp > on entry to a DSO in 32-bit files, is > > not supported in ELF-64 or in the new 32-bit ABI. Instead, these relocations > should be composed with > > R_MIPS_GPREL applied to an explicit symbol for the entry point of the > subprogram I'm not sure what document you're quoting, but sometimes these things express assumptions about a particular usage case that don't apply to what musl is doing. But you should be able to avoid writing this code anyway. > involves loading the difference between the entry > > address of a subprogram s and the runtime gp for establishing gp: > > A: lui rx, %hi(%neg(%gp_rel(s)))# load high part of diff > > B: daddiu rx, rx, %lo(%neg(%gp_rel(s)))# add low part > > C: dadd gp, t9, rx # add to entry address “ > > > > Now I am able to build dynamic n64 musl libraries and n64 dynamic > application but it’s not showing any relocation with it > > and execution of application is giving errors like “unsupported relocation > type errors” Did you make a reloc.h defining the relocation types needed for mips n64 and mapping them to the generic-reloc names musl uses for them? See the existing arch/*/reloc.h files. Without these being defined directly the dynamic linker will not be able to process the relocations it finds in itself or in the main program or other libraries. But without a working crt_arch.h I'm surprised the dynamic linker can even start at all. Are you sure you're not using the mips32 dynamic linker? Look at the program headers (using readelf -l) for INTERP. > $ objdump -R main > > > > a.out: file format elf64-tradbigmips > > > > DYNAMIC RELOCATION RECORDS (none) > > > > $ ./main > > Error relocating libc.so: unsupported relocation type 482431 > > Error relocating libc.so: unsupported relocation type 482687 > > Error relocating libc.so: unsupported relocation type 482943 > > Error relocating libc.so: unsupported relocation type 483199 > > ; > > ; > > ; > > Segmentation fault > > > > So Can you suggest how can I resolve these errors? I am thinking like > either doing changes in do_mips_reloc(),do_relocs() functions in musl > sources or in usage of relocations. OK, I see what's going wrong here. The invalid relocation types are being produced by this line in do_mips_relocs: rel = sym-p->syms << 8 | R_MIPS_JUMP_SLOT; That produces a relocation in the elf32 encoding, because I just assumes mips is elf32 when I wrote it. I believe the 64-bit version should be: rel = sym-p->syms << 32 | R_MIPS_JUMP_SLOT; Try just making that change and see if it works. If so we can work on making a clean conditional for 32- vs 64-bit here later. Rich
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